The present embodiments relate to producing a polycrystalline ceramic film.
Thin-film bulk acoustic resonators (FBAR), which experience changes in resonant frequency on specific binding of substances to be detected on a surface, are increasingly finding application in the area of biosensors. These are piezoelectric crystal layers that are applied to appropriate substrates (e.g., Si wafers).
Because the detection of biomolecules takes place in fluids, high resonance quality and sensitivity are to be provided. For this, the resonator is to be excited in the acoustic shear mode.
In order to make such excitation possible, the polar crystal axis is to be inclined to the exciting field. In a classical resonator structure, in which the piezoelectric is enclosed between two electrode layers, the polar axis is to therefore have a component in the electrode plane.
When ZnO is used as the dielectric, at tilts of approximately 40° and approximately 90°, pure shear waves are excited (Foster, N. F. et al., Cadmium Sulfide and Zinc Oxide Thin-Film Transducers, IEEE Transactions on sonics and ultrasonics, Vol. SU-15, No. 1, January 1968). However, even relatively small tilts of, for example, 15° are sufficient to obtain functional shear-mode resonators
In order to achieve such a tilt of the polar axis, DE 10 2005 014 160 A1 describes a method in which the piezoelectric is deposited by reactive sputtering. Using a diaphragm, a preferred direction of incidence of the particles is established, which makes the desired angle with a normal to the substrate surface on which the ceramic is being deposited.
A disadvantage of the known method is that most of the stream of material is deposited on the diaphragm and not on the substrate. This leads to low deposition rates, and the equipment is to be cleaned and reset after just a few deposition operations. This is very time-consuming and results in high costs. Use of the diaphragm system also results in low reproducibility of the deposition result, especially with respect to layer homogeneity.